Bangkok, Thailand — A bomb explosion Tuesday on a bus in Manila that killed four people is the worst terrorist incident in the Philippine capital in several years. Suspicion is almost certain to fall on Islamic militants blamed for past bombings, including attacks on public transport.

A police official told Philippine media that a homemade bomb had been placed under the seat of the bus, which was carrying commuters on a busy city avenue. A further 14 people were injured in the blast, which tore a hole in the side of the bus and in nearby concrete buildings.

Abu Sayyaf, an Islamist group based in the southern Philippines, is believed to have planted a similar bomb on a city bus in 2005. It was also blamed for bombing a ferry in 2004, killing more than 100 people. In recent years, though, it has operated mostly on its home turf where Philippine troops trained and equipped by US Special Forces are deployed.

In Tuesday’s aftermath, cellphone messages in Manila relayed rumors of a destabilization plot against President Benigno Aquino, who took power last year. But coup rumors are common in the Philippines and particularly after bomb attacks, and there have been no reports of troop movements. The entire police force was put on alert Tuesday.

Western embassies in Manila have reportedly been on high alert in recent months. But Philippine officials have downplayed the threat and said last year that militant groups weren’t capable of launching terrorist attacks in the capital.

Investigators will likely try to match the bomb’s composition with those used by militant groups in the south, where bombings are more common and often incorporate mortar shells. Apart from Abu Sayyaf, the southern Philippines plays host to the Moro Islamic Liberation Front, which is in peace talks with the government, as well as other smaller groups, mostly criminal in nature.